Not Applicable.
The present embodiments relate to telecommunications systems, and are more particularly directed to circuits, systems, and methods for error elimination in a digital subscriber line modem.
The exchange of digital information between remotely located computers is now a pervasive part of modem computing, and occurs in all sorts of contexts including business, education, and personal computer use. In addition, such uses by all current predictions appear to be even more desirable in the future. Video on demand (xe2x80x9cVODxe2x80x9d) is one area which has for some time driven the advancement of technology in this area. More recently, the rapid increase in use and popularity of the Global Interet (hereafter, the xe2x80x9cInternetxe2x80x9d) has perhaps surpassed the excitement created by VOD. This Internet focus also has further motivated research and preliminary development of systems directed to advanced communication of information between remotely located computers. These factors as well as the continuing evolution of computer information exchange give rise to the present embodiments.
One type of technology arising from the above and continuing to evolve is referred to in the art as digital subscriber line (xe2x80x9cDSLxe2x80x9d). DSL is a public network technology that delivers relatively high bandwidth over conventional telephone company copper wiring at limited distances. As explored briefly below, DSL has been further separated into several different categories. All of these differing DSL categories are currently developing, some at different rates than others. In any event, the evolution prevents an absolute definition of any specific DSL category, but some observations may be made at the current time and are explored below.
Given the various DSL technology categories, it may be stated that each differs in some respects while each also shares some similarities. As to differences of the DSL categories, they may diverge in one or more of the expected data transfer rate, the medium type and length over which data are communicated, and the scheme for encoding and decoding data for communication. As to the similarities of the DSL technologies, generally speaking each DSL system is provisioned into modem pairs. One modem of the modem pair is located at a customer site. The other modem of the modem pair is located at the site of an owner, or controller, of a twisted conductor pair network. Currently, the most evident owner or controller is a telephone company central office. Within the telephone company system, its modem is connected to communicate with some type of network, often referred to as a backbone network. The backbone network is further coupled in a network manner to provide other communication paths to and from the backbone network. These other paths are often accomplished through the use of routers, and more recently it has been proposed to eliminate routers in favor of a hardware device referred to as a digital subscriber line access multiplexer (xe2x80x9cDSLAMxe2x80x9d). In any event, given its network nature, the backbone network may further communicate with other information sources and, most notably under current technology, with the Internet. Thus, information accessible to the backbone network, such as Internet information, may be communicated between the central office DSL modem and a customer site with its own compatible DSL modem. Within this general system, it is also anticipated that data rates between DSL modems may be far greater than current voice modem rates. Indeed, current DSL systems being tested or projected range in rates on the order of 500 Kbps to 18 Mbps, or even faster. As explored briefly below, however, note that the higher rates for some DSL systems are only for so-called downstream communications, that is, from the central office to the customer site; thus, for those systems, communication in the other direction (i.e., upstream from the customer site to the central office) is generally at a rate considerably lower than the downstream rate. Lastly, note that most DSL technologies do not use the whole bandwidth of the twisted pair and, thus, often reserve low bandwidth for a voice channel. As a result, while a line is being used by a DSL system, the same line may concurrently communicate a voice conversation as well.
Briefly looking at perhaps the most publicized DSL technology currently being developed, it is referred to as Asymmetric Digital Subscriber Line, or xe2x80x9cADSL.xe2x80x9d ADSL has been standardized by ANSI as seen by its T1.413 standard. However, even given that standard, there continues to be debate and competition as to whether devices complying with the standard provide promise for future wide scale use, and indeed whether the standard requires revision. For example, the standard currently contemplates a modulation technology called Discrete Multitone (DMT) for the transmission of high speed data, but it could be that the standard may further include alternative data transmission techniques in the future. In any event, given the state of the art discussion of ADSL systems, it is contemplated that they will communicate over a single copper twisted pair, and provide downstream rates on the order of 1.5 Mbps to 9 Mbps, while upstream bandwidth will range from 16 kbps to 640 kbps. Along with Internet access, telephone companies are considering delivering remote local area network (xe2x80x9cLANxe2x80x9d) access and VOD services via ADSL.
As to other DSL categories being developed, they include High-Bit-Rate Digital Subscriber Line (xe2x80x9cHDSLxe2x80x9d), Single-Line Digital Subscriber Line (xe2x80x9cSDSLxe2x80x9d), and Very-high-data-rate Digital Subscriber Line (xe2x80x9cVDSLxe2x80x9d). HDSL, unlike ADSL as described above, has a symmetric data transfer rate, that is, it communicates at the same speed in both the upstream and downstream directions. Current perceived speeds are on the order of 1.544 Mbps of bandwidth, but require two copper twisted pairs. HDSL""s operating range is more limited than that of ADSL, and is currently considered to be effective at distances of approximately 12,000 feet. Beyond such a distance, HDSL communication requires signal repeaters to extend the service. SDSL delivers a comparable speed and also a symmetric data transfer as compared to HDSL, but achieves these results with a single copper twisted pair. However, the operating range of an SDSL system is limited to approximately 10,000 feet. Lastly, VDSL provides asymmetric data transfer rates, but anticipates much higher speeds than those competing DSL technologies described above. Currently, rates over a single twisted copper pair on the order of 13 Mbps to 52 Mpbs downstream, and 1.5 Mbps to 2.3 Mbps upstream, are contemplated. Note, however, that such rates are expected to operate only over a range of 1,000 to 4,500 feet.
Given the many variations of DSL technology as introduced above, it is known that for an effective communication system there must be considerations directed to error reduction or elimination in the communication between DSL modems. More specifically, DSL modems typically perform some type of error detection during initialization, and based on the detected errors make adjustments so that post-initialization communications operate to avoid these errors. For example, under the current ADSL standard, DMT modulation is used for the transmission of high speed data, and during initialization there is a determination of xe2x80x9cbit loading,xe2x80x9d that is, how bits are allocated to the different frequency channels of the communication. This bit loading, therefore, may be corrected due to detected errors during initialization. However, for this or alternative approaches, it is recognized in connection with the present inventive embodiments that different errors (e.g., noise) may occur in modem operation after initialization, that is, new noises may arise which were not present during initialization. Accordingly, these noises may cause errors in the data transmitted between modems. Still further, the effect of these noises many not be constant; for example, noises may start and stop, and may change frequency.
From the above, it is further recognized that DSL modems must provide error correction functionality. In this regard, various DSL technologies under development contemplate differing approaches. For example, some DSL modems propose a Reed-Solomon integrated circuit Still others implement some other application specific integrated circuit (xe2x80x9cASICxe2x80x9d) to separately perform an error correction capability. In either of these approaches, however, the benefit from the correction must be measured against its costs. For example, the complexity and added hardware to implement these approaches may outweigh the benefit of the correction technology. As another example, these approaches are heavily constrained in hardware. Thus, if a change in the approach is thereafter desired, the fixed hardware approach may be rendered useless if it will not accommodate a new or changed approach. As another and perhaps most important example for a technology having such broad potential dissemination, extreme complexity may increase the cost of implementing the technology to an unacceptable level. Given the above, the present inventors set forth below various inventive embodiments which seek to overcome the discussed drawbacks as well as others which may be ascertained by one skilled in the art
In one preferred embodiment, there is a method of communicating bit groups between a first DSL modem and a second DSL modem. The method receives at the first DSL modem a plurality of bit groups from the second DSL modem. Each of the plurality of bit groups comprises a plurality of bit sub-groups, and each of the plurality of bit sub-groups has an order of location. For each of the plurality of bit groups, the method determines whether bits of at least one sub-group having a common order of location comprise at least one erroneous bit. In response to this determination, the method stores in the first DSL modem a record of whether the at least one sub-group having the common order of location comprises at least one erroneous bit. From this record, various other steps may be taken in the preferred embodiment, including suppressing the at least one sub-group from communicating valid information from the second DSL modem to the first DSL modem. Other circuits, systems, and methods are also disclosed and claimed.